The present invention relates to fastener-driving tools which are typically powered by combustion, pneumatics, electricity or powder. In such tools, a plurality of fasteners are sequentially arranged in a magazine and are urged by a biased follower toward a driving end of the magazine where the fasteners are each pushed into a nosepiece. Once in the nosepiece, the fasteners are driven into a workpiece by a reciprocating driver blade.
A design criterion of most such tools is that the driving blade should be immobilized when the magazine is empty of fasteners. So-called xe2x80x9cdry firingxe2x80x9d can damage the workpiece or the tool itself Indicator mechanisms are known in such tools to indicate to the user when the magazine is empty or almost empty, so that the magazine can be refilled prior to a dry firing condition. In some of these known mechanisms, the driver blade is locked when the magazine is empty to prevent dry firing.
The issue of prevention of dry firing is particularly challenging when the fasteners are finish nails, which typically are provided in stamped strips. The problems associated with driving such fasteners are described in commonly-assigned U.S. Pat. No. 6,176,412 which is incorporated by reference. Each fastener is relatively thin, and as the fasteners become smaller, they also become thinner. This reduced thickness results in a relatively small increment of movement of the follower upon the driving of each fastener. Thus, it is difficult to design a tool to precisely monitor a fixed number of remaining fasteners to indicate when the magazine needs refilling. This problem is exacerbated by the fact that a given fastener-driving tool is often designed to accommodate a variety of fastener sizes. If, for example, the tool is designed so that a visual, audible or tactile warning is generated upon there being only ten fasteners left in the magazine, the location of the end of the particular strip of fasteners will vary with the size of the fastener. When very thin fasteners are used, such as small finish nails, the combined length of the ten fasteners may be insufficient to reliably enable a warning system to warn the user in time to prevent dry firing.
Thus, it is a first object of the present invention to provide an improved lockout mechanism for a fastener-driving tool which prevents dry firing for a variety of fastener sizes.
Another object of the present invention is to provide an improved lockout mechanism which triggers the lockout function through a mechanical magnification of the significance of a single fastener thickness.
Still another object of the present invention is to provide an improved lockout mechanism which prevents dry firing without requiring additional parts and assembly to the existing tool.
The above-listed objects are met or exceeded by the present lockout mechanism, which features a bypass structure in a fixed position on the magazine near the driver end of the magazine adjacent the nosepiece. Once the number of fasteners in the magazine is reduced so that the follower reaches the bypass structure, the follower becomes disengaged from the remaining fasteners, and moves toward the nosepiece to lock the tool and prevent firing.
More specifically, the present invention provides a lockout mechanism for a fastener-driving tool having a magazine for storing and sequentially urging fasteners toward a nosepiece through which a driver blade impacts and drives the fasteners into a workpiece. The lockout mechanism is configured for preventing the firing of the fastener-driving tool when the magazine is empty or nearly empty of fasteners. A fastener track is defined in the magazine for the passage of the fasteners toward the nosepiece. A biased follower urges the fasteners in the fastener track toward the nosepiece. The fastener track has a bypass portion dimensioned for receiving the follower, whereby upon reaching the bypass portion, the follower becomes disengaged from the fasteners and moves into engagement with the nosepiece for preventing operation of the tool. The bypass portion is located near a magazine driving end to indicate when the magazine is empty or nearly empty.